CHAPTER 1 INTRODUCTION Electr Electrical ical power power is generat generated, ed, transm transmitt itted ed in the form form of alterna alternating ting current. current. The electr electric ic power power produced at the power stations stations is delivered delivered to the the consumers consumers through through a large network network of transmiss transmission ion & distribu distributio tion. n. The transm transmiss ission ion network network is inevita inevitable ble long and high high power power lines lines are necessa necessary ry to maintain a huge block of power source of generation to the load centers to inter connected. Power house for increased reliability of supply greater. The assembly of apparatus used to change some characteristics (e.g. voltage, ac to dc, frequency, power factor etc.) of electric supply keeping the power constant is called a substation. An electrical substation is a subsidiary station of an electricity generation, transmission and distribution system where voltage is transformed from high to low or the reverse using transformers. Electric power may flow through several substations between generating plant and consumer, and may be changed in voltage in several steps.
Fig.1 - 220 KV GSS IG Nagar
1
Substations have switching, protection and control equipment and one or more transformers. In a large substation, circuit breaker are used to interrupt any short-circuits or overload currents that may occur on the network. Depending on the constructional feature, the high voltage substations may be further subdivided:
(a) Outdoor Outdoor substa substatio tion n (b) Indoor Indoor subst substatio ation n (c) Base or Undergro Underground und substat substation ion
1.1) 220KV Grid Substation, IG Nagar: Its part of RRVPNL. It is situated 13.4km away from Jaipur. The power mainly comes from three incoming feeders of 220 KV(Heerapura). The substation is equipped with various equipments and there are various arrangements arrangements for the protection protection purpose. The equipments equipments in the GSS are listed previously. At this substation following feeders are established. 1. TIE FEEDERS 2.
RAD RADIAL IAL FEE FEED DERS ERS
220KV GSS IG Nagar is an outdoor type primary substation and distribution as well it has not only step down but the distribution work The electrical work in a substation comprises to: 1. Choice Choice of bus bus bar bar arra arrangem ngement ent layou layout. t. 2. Selecti Selection on of of rati rating ng of isolator isolator.. 3. Selecti Selection on of rating rating of instr instrume ument nt transfo transforme rmer. r. 4. Sele Selecti ction on of of rati rating ng of of C.B C.B.. 5. Selection of lighting arrester [LA]
6. Selecti Selection on of rati rating ng of powe powerr transf transform ormer er 7. Selection Selection of protective protective relaying relaying scheme, control and relay relay boards. boards. 8. Selecti Selection on of volt voltage age regul regulator ator equipme equipment. nt. 9. Design Design a layout layout of earthing grids and and protection protection against against lightening lightening stockes. stockes.
1.2
INCOMING FEEDERS: 2
Substations have switching, protection and control equipment and one or more transformers. In a large substation, circuit breaker are used to interrupt any short-circuits or overload currents that may occur on the network. Depending on the constructional feature, the high voltage substations may be further subdivided:
(a) Outdoor Outdoor substa substatio tion n (b) Indoor Indoor subst substatio ation n (c) Base or Undergro Underground und substat substation ion
1.1) 220KV Grid Substation, IG Nagar: Its part of RRVPNL. It is situated 13.4km away from Jaipur. The power mainly comes from three incoming feeders of 220 KV(Heerapura). The substation is equipped with various equipments and there are various arrangements arrangements for the protection protection purpose. The equipments equipments in the GSS are listed previously. At this substation following feeders are established. 1. TIE FEEDERS 2.
RAD RADIAL IAL FEE FEED DERS ERS
220KV GSS IG Nagar is an outdoor type primary substation and distribution as well it has not only step down but the distribution work The electrical work in a substation comprises to: 1. Choice Choice of bus bus bar bar arra arrangem ngement ent layou layout. t. 2. Selecti Selection on of of rati rating ng of isolator isolator.. 3. Selecti Selection on of rating rating of instr instrume ument nt transfo transforme rmer. r. 4. Sele Selecti ction on of of rati rating ng of of C.B C.B.. 5. Selection of lighting arrester [LA]
6. Selecti Selection on of rati rating ng of powe powerr transf transform ormer er 7. Selection Selection of protective protective relaying relaying scheme, control and relay relay boards. boards. 8. Selecti Selection on of volt voltage age regul regulator ator equipme equipment. nt. 9. Design Design a layout layout of earthing grids and and protection protection against against lightening lightening stockes. stockes.
1.2
INCOMING FEEDERS: 2
1.3
•
220 KV(Heerapura)
•
220 KV Bassi(PGCIL)
•
Puranaghat(Currently Dead)
OUTGOING FEEDERS:
The outgoing feeders are: 1. 2.
Puranaghat 220 KV Jagatpura 33 KV
3. Mahel ahel 33 KV KV 4. IG Nag Nagar 33 KV 5. Bisa Bisalp lpur ur 33 KV 6. Prad Pradha han n Mar Marg g 33 33 KV KV Rajasthan Rajya Vidyut Prasaran Nigam Limited (RRVPNL) a company under the Companies Act, 1956 and registered with Registrar of Companies as "RAJASTHAN RAJYA VIDYUT PRASARAN NIGAM LIMITED" LIMITED" vide No. 17-016485 17-016485 of 2000-2001 2000-2001 with its Registered Registered Office at VIDYUT VIDYUT BHAWAN, JYOTI NAGAR, JAIPUR-302005 has been established on 19 July, 2000 by Govt. of Rajast Rajasthan han under the provisio provisions ns of the Rajastha Rajasthan n Power Power Sector Sector Reform Reform act 1999 as the succes successor sor company of RSEB. The RERC has granted RRVPNL a license for transmission and bulk supply vide RERC/Transmission and Bulk Supply License 4/2001 dated 30. Our aim is to provide reliable electric transmission service to these customers. As a public utility whose infrastructure serves as the link in transporting electricity to millions of electricity users, RRVPNL has following duties and responsibilities: • Intra state transmission of electricity through Intra-State Transmission System. • Ensuring development of an efficient, co-ordinated and economical system of intra-state transmission of electricity from generating stations to Load Centers. • Non-discriminatory Open Access to its transmission system on payment of transmission charges • Complying with the directions of RLDC and SLDC, operating SLDC until any other authority is established by the State Govt. • Now RRVPNL is "An ISO 9001:2000 Certified Company".
3
4
5
Fig. 2: Single Line Diagram of 220KV GSS Heerapura(Jaipur)
CHAPTER 2 LIGHTNING ARRESTER
Fig. 3- Lightning arrester
A lightning arrester (also known as surge diverter) is a device connected between line and earth i.e. in parallel with the over headline, HV equipments and substation to be protected. It is a safety valve which limits the magnitude of lightning and switching over voltages at the substations, over headlines and HV 6
equipments and provides a low resistance path for the surge current to flow to the ground. The practice is also to install lightning arresters at the incoming terminals of the line. All the electrical equipments must be protected from the severe damages of lightning strokes. The techniques can be studied under:
Protection of transmission line from direct stroke.
Protection of power station and sub-station from direct stroke.
Protection of electrical equipments from travelling waves.
2.1) Types of Arrestors:2.1.1)
Rod/sphere gap:- It is a very simple protective device i.e. gap is provided
across the stack of Insulators to permit flash-over when undesirable voltages are impressed of the system. 2.1.2)
Expulsion type LA:- It have two electrodes at each end and consists of a fiber
tube capable of producing a gas when is produced. The gas so evolved blows the arc through the bottom electrode.
2.1.3) Valve type LA:- It consists of a divided spark-gap in series will a non linear resistor. The divided spark gap consists of a no. of similar elements, each of it two electrode across which are connected high resistor.
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CHAPTER 3 BUS BARS Bus Bars are the common electrical component through which a large no of feeders operating at same voltage have to be connected. If the bus bars are of rigid type (Aluminum types) the structure height are low and minimum clearance is required. While in case of strain type of bus bars suitable ACSR conductor are strung/tensioned by tension insulators discs according to system voltages. In the widely used strain type bus bars stringing tension is about 500-900 Kg depending upon the size of conductor used. Here proper clearance would be achieved only if require tension is achieved. Loose bus bars would effect the clearances when it swings while over tensioning may damage insulators. Clamps or even effect the supporting structures in low temperature conditions. The clamping should be proper, as loose clamp would spark under in full load condition damaging the bus bars itself.
3.1) BUS BAR ARRENGEMENT MAY BE OF FOLLOWING TYPE WHICH IS BEING ADOPTED BY R.R.V.P.N.L.:3.1.1) Single bus bar arrangement 3.1.2) Double bus bar arrangement a) Main bus with transformer bus b) Main bus-I with main bus-II 3.1.3) Double bus bar arrangement with auxiliary bus.
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3.1.1) SINGLE BUS BAR ARRANGEMENT : This arrangement is simplest and cheapest. It suffers, however, from major defects. 1. Maintenance without interruption is not possible. 2. Extension of the sub station without a shut down is not possible
3.1.2) DOUBLE BUS BAR ARRANGEMENT : 1. Each load may be fed from either bus. 2. The load circuit may be divided in to two separate groups if needed from operational consideration. Two supplies from different sources can be put on each bus separately. 3. Either bus bar may be taken out from maintenance of insulators. The normal bus selection insulators can not be used for breaking load currents. The arrangement does not permit breaker maintenance without causing stoppage of supply.
3.1.3) DOUBLE BUS BAR ARRANGEMENTS CONTAINS MAIN BUS WITH AUXILARY BUS : The double bus bar arrangement provides facility to change over to either bus to carry out maintenance on the other but provide no facility to carry over breaker maintenance. The main and transfer bus works the other way round. It provides facility for carrying out breaker maintenance but does not permit bus maintenance. Whenever maintenance is required on any breaker the circuit is changed over to the transfer bus and is controlled through bus coupler breaker.
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CHAPTER 4 INSULATOR The insulator for the overhead lines provides insulation to the power conductors from the ground so that currents from conductors do not flow to earth through supports. The insulators are connected to the cross arm of supporting structure and the power conductor passes through the clamp of the insulator. The insulators provide necessary insulation between line conductors and supports and thus prevent any leakage current from conductors to earth. In general, the insulator should have the following desirable properties: •
High mechanical strength in order to withstand conductor load, wind load etc.
•
High electrical resistance of insulator material in order to avoid leakage currents to earth.
•
High relative permittivity of insulator material in order that dielectric strength is high.
•
High ratio of puncture strength to flash over.
These insulators are generally made of glazed porcelain or toughened glass. Poly come type insulator [solid core] are also being supplied in place of hast insulators if available indigenously. The design of the insulator is such that the stress due to contraction and expansion in any part of the insulator does not lead to any defect. It is desirable not to allow porcelain to come in direct contact with a hard metal screw thread.
4.1) TYPE OF INSULATORS: 4.1.1: Pin type 4.1.2: Suspension type 4.1.3: Strain insulator 4.1.1)
PIN TYPE: Pin type insulator consist of a single or multiple shells adapted to
be mounted on a spindle to be fixed to the cross arm of the supporting structure. When the upper most shell is wet due to rain the lower shells are dry and provide sufficient leakage resistance these are used for transmission and distribution of electric power at 10
voltage up to voltage 33 KV. Beyond operating voltage of 33 KV the pin type insulators thus become too bulky and hence uneconomical.
Fig. 4-Pin type insulator
4.1.2)
SUSPENSION TYPE:
Suspension type insulators consist of a number of
porcelain disc connected in series by metal links in the form of a string. Its working voltage is 66KV. Each disc is designed for low voltage for 11KV.
11
Fig. 5-Suspension type insulator 4.1.3)
STRAIN INSULATOR: The strain insulators are exactly identical in shape
with the suspension insulators. These strings are placed in the horizontal plane rather than the vertical plane. These insulators are used where line is subjected to greater tension. For low voltage lines (< 11KV) shackle insulator are used as strain insulator.
Fig. 6-Strain type insulator
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CHAPTER 5 ISOLATORS “Isolator" is one, which can break and make an electric circuit in no load condition. These are normally used in various circuits for the purposes of Isolation of a certain portion when required for maintenance etc. Isolation of a certain portion when required for maintenance etc. "Switching Isolators" are capable of •
Interrupting transformer magnetized currents
•
Interrupting line charging current
•
Load transfer switching
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Fig. 7- Isolators
Its main application is in connection with transformer feeder as this unit makes it possible to switch out one transformer, while the other is still on load. The most common type of isolators is the rotating centre pots type in which each phase has three insulator post, with the outer posts carrying fixed contacts and connections while the centre post having contact arm which is arranged to move through 90` on its axis. The following interlocks are provided with isolator: a) Bus 1 and2 isolators cannot be closed simultaneously. b) Isolator cannot operate unless the breaker is open. c) Only one bay can be taken on bypass bus. d) No isolator can operate when corresponding earth switch is on breaker.
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CHAPTER 6 CIRCUIT BREAKER The function of relays and circuit breakers in the operation of a power system is to prevent or limit damage during faults or overloads, and to minimize their effect on the remainder of the system. This is accomplished by dividing the system into protective
zones separated by circuit breakers. During a
fault, the zone which includes the faulted apparatus is de-energized and disconnected from the system. In addition to its protective function, a circuit breaker is also used for circuit switching under normal conditions. Each having its protective relays for determining the existence of a fault in that zone and having circuit breakers for disconnecting that zone from the system. It is desirable to restrict the amount of system disconnected by a given fault; as for example to a single transformer, line section, machine, or bus 15
section. However, economic considerations frequently limit the number of circuit breakers to those required for normal operation and some compromises result in the relay protection. Some of the manufacturers are ABB, AREVA, Cutler-Hammer (Eaton), Mitsubishi Electric, Pennsylvania Breaker, Schneider Electric, Siemens, Toshiba, Končar HVS and others. Circuit breaker can be classified as "live tank", where the enclosure that contains the breaking mechanism is at line potential, or dead
tank with
the enclosure at earth potential. High-voltage AC
circuit breakers are routinely available with ratings up to 765,000 volts.
6.1) Various types of circuit breakers:6.1.1) SF6 Circuit Breaker 6.1.2) Air Blast Circuit Breaker 6.1.3) Oil Circuit Breaker 6.1.4) Bulk Oil Circuit Breaker (MOCB) 6.1.5) Minimum Oil Circuit Breaker
6.1.1) SF6 CIRCUIT BREAKER:Sulphur hexafluoride has proved its-self as an excellent insulating and arc quenching medium. It has been extensively used during the last 30 years in circuit breakers, gas-insulated switchgear (GIS), high voltage capacitors, bushings, and gas insulated transmission lines. In SF6 breakers the contacts are surrounded by low pressure SF6 gas. At the moment the contacts are opened, a small amount of gas is compressed and forced through the arc to extinguish it.
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Fig. 8-SF6 Circuit Breaker 6.1.2) AIR BLAST CIRCUIT BREAKER:
The principle of arc interruption in air blast circuit breakers is to direct a blast of air, at high pressure and velocity, to the arc. Fresh and dry air of the air blast will replace the ionized hot gases within the arc zone and the arc length is considerably increased. Consequently the arc may be interrupted at the first natural current zero. In this type of breaker, the contacts are surrounded by compressed air. When the contacts are opened the compressed air is released in forced blast through the arc to the atmosphere extinguishing the arc in the process.
17
Fig. 9-Air Blast Circuit Breaker
Advantages:
An air blast circuit breaker has the following advantages over an oil circuit breaker: •
The risk of fire is eliminated
•
The arcing products are completely removed by the blast whereas the oil deteriorates with successive operations; the expense of regular oil is replacement is avoided
•
The growth of dielectric strength is so rapid that final contact gap needed for arc extinction is very small. this reduces the size of device
•
The arcing time is very small due to the rapid build up of dielectric strength between contacts. Therefore, the arc energy is only a fraction that in oil circuit breakers, thus resulting in less burning of contacts
•
Due to lesser arc energy, air blast circuit breakers are very suitable for conditions where frequent operation is required
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•
The energy supplied for arc extinction is obtained from high pressure air and is independent of the current to be interrupted.
Disadvantages:
Air has relatively inferior arc extinguishing properties. •
Air blast circuit breakers are very sensitive to the variations in the rate of restriking voltage.
•
Considerable maintenance is required for the compressor plant which supplies the air blast
•
Air blast circuit breakers are finding wide applications in high voltage installations. Majority of circuit breakers for voltages beyond 110 kV are of this type.
6.1.3) OIL CIRCUIT BREAKER: Circuit breaking in oil has been adopted since the early stages of circuit breakers manufacture. The oil in oil-filled breakers serves the purpose of insulating the live parts from the earthed ones and provides an excellent medium for arc interruption. Oil circuit breakers of the various types are used in almost all voltage ranges and ratings. However, they are commonly used at voltages below 115KV leaving the higher voltages for air blast and SF6 breakers. The contacts of an oil breaker are submerged in insulating oil, which helps to cool and extinguish the arc that forms when the contacts are opened. Oil circuit breakers are classified into two main types namely: bulk oil circuit breakers and minimum oil circuit breakers. The advantages of using oil as an arc quenching medium are: 1. It absorbs the arc energy to decompose the oil into gases, which have excellent cooling properties. 2. It acts as an insulator and permits smaller clearance between live conductors and earthed components. The disadvantages of oil as an arc quenching medium are: 1. Its inflammable and there is risk of fire 2. It may form an explosive mixture with air. 3. The arcing products remain in the oil and it reduces the quality of oil after several operations. 4. This necessitates periodic checking and replacement of oil.
6.1.4) BULK OIL CIRCUIT BREAKER: 19
Bulk oil circuit breakers are widely used in power systems from the lowest voltages up to 115KV. However, they are still used in the systems having voltages up to 230KV. The contacts of bulk oil breakers may be of the plain-break type, where the arc is freely interrupted in the oil, or enclose within the arc controllers. Plain-break circuit breakers consist mainly of a large volume of oil contained in a metallic tank. Arc interruption depends on the head of oil above the contacts and the speed of contact separation. The head of oil above the arc should be sufficient to cool the gases, mainly hydrogen, produced by oil decomposition. A small air cushion at the top of the oil together with the produced gases will increase the pressure with a subsequent decrease of the arcing time.
6.1.5) MINIMUM OIL CIRCUIT BREAKER: Bulk oil circuit breakers have the disadvantage of using large quantity of oil. With frequent breaking and making heavy currents the oil will deteriorate and may lead to circuit breaker failure. This has led to the design of minimum oil circuit breakers working on the same principles of arc control as those used in bulk oil breakers. In this type of breakers the interrupter chamber is separated from the other parts and arcing is confined to a small volume of oil. The lower chamber contains the operating mechanism and the upper one contains the moving and fixed contacts together with the control device. Both chambers are made of an insulating material such as porcelain. The oil in both chambers is completely separated from each other. By this arrangement the amount of oil needed for arc interruption and the clearances to earth are roused. However, conditioning or changing the oil in the interrupter chamber is more frequent than in the bulk oil breakers. This is due to carbonization and slugging from arcs interrupted chamber is equipped with a discharge vent and silica gel breather to permit a small gas cushion on top of the oil. Single break minimum oil breakers are available in the voltage range 13.8 to 34.5 KV.
CHAPTER 7 PROTECTIVE RELAYS 20
Relays must be able to evaluate a wide variety of parameters to establish that corrective action is required. Obviously, a relay cannot prevent the fault. Its primary purpose is to detect the fault and take the necessary action to minimize the damage to the equipment or to the system. The most common parameters which reflect the presence of a fault are the voltages and currents at the terminals of the protected apparatus or at the appropriate zone boundaries. The fundamental problem in power system protection is to define the quantities that can differentiate between normal and abnormal conditions. This problem is compounded by the fact that “normal” in the present sense means outside the zone of protection. This aspect, which is of the greatest significance in designing a secure relaying system, dominates the design of all protection systems.
Fig. 10-Relays
7.1 ) Distance Relays: Distance relays respond to the voltage and current, i.e., the impedance, at the relay location. The impedance per mile is fairly constant so these relays respond to the distance between the relay location and the fault location. As the power systems become more complex and the fault current varies with 21
changes in generation and system configuration, directional over current relays become difficult to apply and to set for all contingencies, whereas the distance relay setting is constant for a wide variety of changes external to the protected line.
7.2) Types of Distance relay:7.2.1) Impedance Relay:
The impedance relay has a circular characteristic centred. It is non directional and is used primarily as a fault detector . 7.2.2) Admittance Relay:
The admittance relay is the most commonly used distance relay. It is the tripping relay in pilot schemes and as the backup relay in step distance schemes. In the electromechanical design it is circular, and in the solid state design, it can be shaped to correspond to the transmission line impedance. 7.2.3) Reactance Relay:
The reactance relay is a straight-line characteristic that responds only to the reactance of the protected line. It is non directional and is used to supplement the admittance relay as a tripping relay to make the overall protection independent of resistance. It is particularly useful on short lines where the fault arc resistance is the same order of magnitude as the line length.
CHAPTER 8 POWER TRANSFORMER 8.1) Windings: 22
Winding shall be of electrolytic grade copper free from scales & burrs. Windings shall be made in dust proof and conditioned atmosphere. Coils shall be insulated that impulse and power frequency voltage stresses are minimum. Coils assembly shall be suitably supported between adjacent sections by insulating spacers and barriers. Bracing and other insulation used in assembly of the winding shall be arranged to ensure a free circulation of the oil and to reduce the hot spot of the winding. All windings of the transformers having voltage less than 66 kV shall be fully insulated. Tapping shall be so arranged as to preserve the magnetic balance of the transformer at all voltage ratio. All leads from the windings to the terminal board and bushing shall be rigidly supported to prevent injury from vibration short circuit stresses.
Fig. 11-Power Transformer
8.2) Tanks and fittings: Tank shall be of welded construction & fabricated from tested quality low carbon steel of adequate thickness. After completion of welding, all joints shall be subjected to dye penetration testing. At least two adequately sized inspection openings one at each end of the tank shall be provided for easy access to bushing & earth connections. Turrets & other parts surrounding the conductor of individual 23
phase shall be non-magnetic. The main tank body including tap changing compartment, radiators shall be capable of withstanding full vacuum.
8.3) Cooling Equipments: Cooling equipment shall conform to the requirement stipulated below: (a.) Each radiator bank shall have its own cooling fans, shut off valves at the top and bottom (80mm size) lifting lugs, top and bottom oil filling valves, air release plug at the top, a drain and sampling valve and thermometer pocket fitted with captive screw cap on the inlet and outlet. (b.) Cooling fans shall not be directly mounted on radiator bank which may cause undue vibration. These shall be located so as to prevent ingress of rain water. Each fan shall be suitably protected by galvanized wire guard.
Fig. 12-Radiator with fan
8.4) Transformer Accessories: 8.4.1) Buchholz Relay:
This has two Floats, one of them with surge catching baffle and gas collecting space at top. This is mounted in the connecting pipe line between conservator and main tank. This is the most dependable protection for a given transformer.
24
Gas evolution at a slow rate that is associated with minor faults inside the transformers gives rise to the operation or top float whose contacts are wired for alarm. There is a glass window with marking to read the volume of gas collected in the relay. Any major fault in transformer creates a surge and the surge element in the relay trips the transformer. Size of the relay varies with oil volume in the transformer and the mounting angle also is specified for proper operation of the relay.
Fig. 13-Buchholz Relay 8.4.2) Temperature Indicators:
Most of the transformer (small transformers have only OTI) are provided with indicators that displace oil temperature and winding temperature. There are thermometers pockets provided in the tank top cover which hold the sensing bulls in them. Oil temperature measured is that of the top oil, where as the winding temperature measurement is indirect.
This is done by adding the temperature rise due to the heat produced in a heater coil (known as image coil) when a current proportional to that flowing in windings is passed in it to that or top oil. For proper functioning or OTI & WTI it is essential to keep the thermometers pocket clean and filled with oil.
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Fig. 14-Winding and oil temperature indicator 8.4.3) Silica Gel Breather:
Both transformer oil and cellulosic paper are highly hygroscopic. Paper being more hygroscopic than the mineral oil The moisture, if not excluded from the oil surface in conservator, thus will find its way finally into the paper insulation and causes reduction insulation strength of transformer. To minimize this conservator is allowed to breathe only through the silica gel column, which absorbs the moisture in air before it enters the conservator air surface.
Fig. 15-Silica gel Breather
8.4.4) Conservator:
With the variation of temperature there is corresponding variation in the oil volume. To account for this, an expansion vessel called conservator is added to the transformer with a connecting pipe to the main tank. In smaller transformers this vessel is open to atmosphere through dehydrating breathers (to keep the air dry). In larger transformers, an air bag is mounted inside
26
the conservator with the inside of bag open to atmosphere through the breathers and the outside surface of the bag in contact with the oil surface.
Fig. 16-Conservator with Buchholz relay and tank [ref.-6]
Total No. of transformers = 6
No. of transformers
220/132 KV------------------------------------ 100MVA
2
132/33 KV--------------------------------------20/25MVA
2
132/33KV---------------------------------------40/50MVA
1
132/11 KV---------------------------------------10/12.5 MVA
1
CHAPTER 9 CURRENT TRANSFORMER
27
As you all know this is the device which provides the pre-decoded fraction of the primary current passing through the line/bus main circuit. Such as primary current 60A, 75A, 150A, 240A, 300A, 400A, to the secondary output of 1A to 5A. Now a day mostly separate current transformer units are used instead of bushing mounting CT’s on leveled structure they should be for oil level indication and base should be earthed properly. Care should be taken so that there should be no strain as the terminals. When connecting the jumpers, mostly secondary connections is taken to three unction boxes where star delta formation is connected for three phase and final leads taken to protection /metering scheme. There should be no chance of secondary circuit remaining opens as it leads to extremely high voltage which ultimately damages the CT itself
Fig. 17-Current Transformers
It can be used to supply information for measuring power flows and the electrical inputs for the operation of protective relays associated with the transmission and distribution circuit or for power transformer. These current transformers have the primary winding connected in series with the conductor carrying the current to be measured or controlled. The secondary winding is thus insulated from the high voltage and can then be connected to low voltage metering circuits. Current transformers are also used for street lighting circuits. Street lighting requires a constant current to prevent flickering lights and a current transformer is used to provide that constant current. In this
28
case the current transformer utilizes a moving secondary coil to vary the output so that a constant current is obtained.
CHAPTER 10 POTENTIAL TRANSFORMER 29
A potential transformer (PT) is used to transform the high voltage of a power line to a lower value, which is in the range of an ac voltmeter or the potential coil of an ac voltmeter.
Fig.18-Potential Transformer
The voltage transformers are classified as under: •
Capacitive voltage transformer or capacitive type
•
Electromagnetic type.
Capacitive voltage transformer is being used more and more for voltage measurement in high voltage transmission network, particularly for systems voltage of 132KV and above where it becomes increasingly more economical. It enables measurement of the line to earth voltage to be made with simultaneous provision for carrier frequency coupling, which has reached wide application in modern high voltage network for tele-metering remote control and telephone communication purpose. The capacitance type voltage transformers are of twp type: •
Coupling Capacitor type
•
Pushing Type
The performance of CVT is affected by the supply frequency switching transient and magnitude of connected Burdon. The CVT is more economical than an electromagnetic voltage transformer when the nominal supply voltage increases above 66KV.
30
The carrier current equipment can be connected via the capacitor of the CVT. There by there is no need of separate coupling capacitor. The capacitor connected in series act like potential dividers, provided, the current taken by burden is negligible compared with current passing through the series connected capacitor.
CVT as coupling capacitor for carrier current application: The carrier current equipments is connected to the power line via coupling capacitor. The coupling CVT combines the function of coupling and voltage transformer.
CHAPTER 11 CAPACITIVE VOLTAGE TRANSFORMER
31
A capacitor voltage transformer (CVT) is a transformer used in power systems to step-down extra high voltage signals and provide low voltage signals either for measurement or to operate a protective relay. In its most basic form the device consists of three parts: two capacitors across which the voltage signal is split, an inductive element used to tune the device to the supply frequency and a transformer used to isolate and further step-down the voltage for the instrumentation or protective relay. The device has at least four terminals, a high-voltage terminal for connection to the high voltage signal, a ground terminal and at least one set of secondary terminals for connection to the instrumentation or protective relay. CVTs are typically single-phase devices used for measuring voltages in excess of one hundred kilovolts where the use of voltage transformers would be uneconomical. In practice the first capacitor, C1, is often replaced by a stack of capacitors connected in series. This results in a large voltage drop across the stack of capacitors that replaced the first capacitor and a comparatively small voltage drop across the second capacitor, C2, and hence the secondary terminals.
The porcelain in multi unit stack, all the potentials points are electrically tied and suitably shielded to overcome the effect of corona RIV etc. Capacitive voltage transformers are available for system voltage.
CHAPTER 12 CONTROL ROOM
32
Control panel contain meters, control switches and recorders located in the control building, also called the dog house. These are used to control the substation equipment to send power from one circuit to another or to open or to shut down circuits when needed.
Fig.19-Control Room in GSS Heerapura
12.1) MEASURING INSTRUMENT USED: 12.1.1)
ENERGY METER: To measure the energy transmitted energy meters are fitted to the
panel to different feeders the energy transmitted is recorded after one hour regularly for it MWHr, meter is provided. 33
12.1.2)
WATTMETERS: It is attached to each feeder to record the power exported from GSS.
12.1.3) FREQUENCY METER: To measure the frequency at each feeder there is the provision of analog or digital frequency meter.
12.1.4) VOLTMETER: It is provided to measure the phase to phase voltage .It is also available in both the analog and digital frequency meter.
12.1.5) AMETER: It is provided to measure the line current. It is also available in both the forms analog as well as digital.
12.1.6) MAXIMUM DEMAND INDICATOR: There are also mounted the control panel to record the average power over successive predetermined period.
12.1.7) MVAR METER: It is to measure the reactive power of the circuit.
CHAPTER 13 CAPACITOR BANK The capacitor bank provides reactive power at grid substation. The voltage regulation problem frequently reduces so of circulation of reactive power. 34
Unlike the active power, reactive power can be produced, transmitted and absorbed of course with in the certain limit, which have always to be workout. At any point in the system shunt capacitor are commonly used in all voltage and in all size.
Fig. 20-Capacitor Bank
Benefits of using the capacitor bank are many and the reason is that capacitor reduces the reactive current flowing in the whole system from generator to the point of installation. 1 .Increased voltage level at the load 2. Reduced system losses 3. Increase power factor of loading current
CHAPTER 14 POWER LINE CARRIER COMMUNICATION As electronics plays a vital role in the industrial growth, communication is also a backbone of any power stations. Communication between various generating and receiving station is very essential for 35
proper operation of power of power system. This is more in case of large interconnected system where a control leads dispatch station has to co-ordinate the working of various unit to see that the system is maintained in the optimum working condition, power line communication is most economic and reliable method of communication for medium and long distance in power network.
14.1) Wave Trap: Line trap also is known as Wave trap. What it does is trapping the high frequency communication signals sent on the line from the remote substation and diverting them to the telecom/teleprotection panel in the substation control room (through coupling capacitor and LMU).
Fig. 21-Wave Trap
This is relevant in Power Line Carrier Communication (PLCC) systems for communication among various substations without dependence on the telecom company network. The signals are primarily teleprotection signals and in addition, voice and data communication signals.
The Line trap
OFFERS HIGH IMPEDANCE TO THE HIGH FREQUENCY COMMUNICATION SIGNALS
thus obstructs the
flow of these signals in to the substation bus bars. If there were not to be there, then signal loss is more and communication will be ineffective/probably impossible.
36
CHAPTER 15 EARTHING OF THE SYSTEM The provision of an earthing system for an electric system is necessary by the following reason. •
In the event of over voltage on the system due to lightening discharge or other system fault. These parts of equipment, which are normally dead, as for as voltage, are concerned do not attain dangerously high potential.
37
•
In a three phase, circuit the neutral of the system is earthed in order to stabilize the potential of circuit with respect to earth.
The resistance of earthing system is depending on: •
Shape and material of earth electrode used.
•
Depth in the soil.
Specific resistance of soil surrounding in the neighbourhood of system electrodes.
15.1) PROCEDURE OF EARTHING: Technical consideration the current carrying path should have enough capacity to deal with more faults current. The resistance of earth and current path should be low enough to prevent voltage rise between earth and neutral. The earth electrode must be driven in to the ground to a sufficient depth to as to obtain lower value of earth resistance. To sufficient lowered earth resistance a number of electrodes are inserted in the earth to a depth, they are connected together to form a mesh. The resistance of earth should be for the mesh in generally inserted in the earth at 0.5m depth the several point of mesh then connected to earth electrode or ground conduction. The earth electrode is metal plate copper is used for earth plate.
15.2) NEUTRAL EARTHING: Neutral earthing of power transformer all power system operates with grounded neutral. Grounding of neutral offers several advantages the neutral point of generator transformer is connected to earth directly or through a reactance in some cases the neutral point is earthed through an adjustable reactor of reactance matched with the line. •
The earth fault protection is based on the method of neutral earthing.
•
The neutral earthing is associated switchgear.
The neutral earthing is provided for the purpose of protection arcing grounds unbalanced
voltages
with respect to protection from lightening and for improvement of the system.
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CHAPTER 16 BATTERY ROOM In a GSS, separate dc supply is maintained for signalling remote position control, alarm circuit etc. Direct current can be obtained from 220volt 3 phase ac supply via rectifier and in event of ac failure, from the fixed batteries, which are kept, charged in normal condition by rectifier supply.
39
Fig. 22-Battery Room
Battery System: The batteries used are lead acid type having a solution of sulphuric acid and distilled water as electrolytes. In charged state, it has a specific gravity of 1.2 at temperature of 30C.In the battery room batteries are mounted on wooden stand. The cells are installed stand by porcelain. Following precautions are taken in a battery room: •
The conductor connecting the cells are greased and coated with electrolyte resisting varnish.
•
Proper care is taken so that acid vapours do not accumulate in the room to avoid risk of explosion, smoking, winding etc.
•
The windows of battery are of forested glass to avoid the batteries from direct action of sun light.
CHAPTER 17 RATINGS
17.1) TRANSFORMER: Total No. of transformers = 6
220/132 KV------------------------------------ 100MVA
No. of transformers
2 40
132/33 KV--------------------------------------20/25MVA
2
132/33KV---------------------------------------40/50MVA
1
132/11 KV---------------------------------------10/12.5 MVA
1
MAKE
Company
220/133 KV, 100MVA X-Mer 1----------------------------------- TELK 220/133KV, 100 MVA
X-Mer 2---------------------------------- ALSTOM
132/33 KV, 20/25 MVA X-Mer 1---------------------------------- TELK 132/33 KV, 20/25 MVA X-Mer 2-----------------------------------BBL 132/33 KV, 40/50 MVA X-Mer 3-----------------------------------T&R 132/33 KV, 10/12.5 MVA X-Mer 1---------------------------------EMCO
17.2) CIRCUIT BREAKER: No. of 220KV breaker
-
6
No. of 132KV breaker
-
13
No. of 33KV breaker
-
12
No. of Capacitor Bank (33kv) -
4
No. of 11KV breaker
7
-
SF6 CB
BREAKER SERIAL NO.
030228
RATED VOLTAGE
145KV
NORMAL CURRENT
1250A
FREQUENCY
5OHz
LIGHTNING IMPULSE WITHSTAND
650KV (Peak)
FIRST POLE TO CLEAR TO CLEAR FACTOR
1-2
SHORT TIME WITHSTAND CURRENT
31.5KA
DURATION OF SHORT CIRCUIT
3 Sec.
(SHORT CIRCUIT
31.5KA
SYM.
41
BREAKING CURRENT)
ASYM.
SHORT TIME MAKING CURRENT
37.5KA 8.0KA
OUT OF PHASE BREAKING CURRENT
7.9KA
OPERATING SEQUENCE
0-0.3-CO-3min-CO
SF6 GAS PRESSURE AT 20C
6.3 Bar
TOTAL MASS OF CB
1300Kg
MASS OF SF6 GAS
8.7Kg
17.3) BATTERY CHARGER: Battery Charger – 220AH
440AH
VDC
HBL NIFE LTD.
VDC
HBL NIFE LTD.
Capacitor BankNo.-1
BHEL
38KV
6.6MVAR
Capacitor BankNo.-2
BHEL
38KV
7.2MVAR
Capacitor BankNo.-1
ABB
38KV
7.2MVAR
Capacitor BankNo.-1
WS
38KV
7.2MVAR
17.4) CURRENT TRANSFORMER : FREQUENCY
50Hz
HIGHEST SYSTEM VOLTAGE
245KV
SHORT TIME CURRENT
40KA/15
RATED CURRENT
600A
CURRENT RATIO
600-300-150/1
MIN. KNEE POTENTIAL VOLTAGE
850V at 150/1
MAX. EXCITING CURRENT
100MA at 150/1
MAX. SEC. WINDING RESISTANCE
2.5OHM at 150/1
17.5) CAPACITIVE VOLTAGE TRANSFORMER: SERIAL NO.
0173537
INSULATION LEVEL
460KV
RATED VOLTAGE FACTOR
1.2/cont
TIME
1.5/30sec.
HIGHEST SYSTEM VOLTAGE
245KV
PRIMARY VOLTAGE
22OKV/1.732 42
TYPE OUTDOOR
Wgt. 850Kg
PHASE SINGLE
TBONP.CAT 50C
SECONDARY VOLTAGE
110/1.732
110/1.732
RATED BURDON
220Va
110Va
FREQUENCY
49.5-50.5Hz
CONCLUSION
Training at 220KV GSS IG Nagar, Jagatpura, Jaipur gives the insight of the real instruments used. There are many instruments like transformer, CT, PT, CVT, LA, relay, PLCC, bus bars, capacitor bank, insulator, isolators, control room, Battery room etc.
What is the various problem seen in substation while handling this instruments.
There are various occasion when relay operate and circuit breaker open, load shedding, shut down, which has been heard previously. 43
To get insight of the substation, how things operate, how things manage all is learned there. Practical training as a whole proved to be extremely informative and experience building and the things learnt at it would definitely help a lot in snapping the future ahead a better way.
BIBILIOGRAPHY
1. B.R.GUPTA (2005), “POWER SYSTEM ANALYSIS AND DESIGN”
P122, P123, S.Chand & Company Ltd.
2. ASHFAQ HUSSAIN ( 2005), “ELECTRICAL POWER SYSTEM” P79, P501, P516,
CBS publisher and distributors.
3. V.K.MEHTA (2002), “POWER SYSTEM” P447, P483, P507, P527, P555,
S.chand & company Ltd. 44
